Studies On The Solution Processing,Modification,and Photovoltaic Properties Of Cu₂ZnSn（S,Se）₄ Semiconductor Films

Cu₂Zn Sn（S,Se）₄（CZTSSe）is a kind of direct band gap material,which has many advantages,including green and earth-abundant constituent elements,high absorption coefficient,adjustable band gap,and so on.Besides,flexible CZTSSe solar cells can be applied to scenarios that cannot be covered by traditional silicon solar cells such as distributed/portable energy and building-integrated photovoltaic.On the other hand,CZTS solar cells can form tandem solar cells with silicon solar cells to further break through the efficiency limit.Therefore,the research on CZTS-based solar cells is of great significance for exploring and developing efficient,low-cost,and environment-friendly solar cells.However,the current world record efficiency of CZTSSe solar cells is only 13%,far below its theoretical conversion efficiency.As the main functional layer of the solar cell,the quality of the CZTSSe absorber layer,including crystallinity,defect characteristics,and carrier transport characteristics,is the key to determining the device performance of kesterite CZTSSe solar cells.Therefore,optimizing the absorber quality and improving the device performance have become the top priority in the kesterite CZTSSe research field.In this dissertation,the following innovative research results are obtained through the research on absorber quality optimization and its photovoltaic performance improvement.（1）Combining the chemical properties of the 2-methoxyethanol solvent（containing hydroxyl groups）,Zn Cl₂,and Sn Cl₂·2H₂O metal salts（hygroscopic）,a solution chemistry strategy via water additive was developed.The introduction of water additive made the complexes based on 2-methoxyethanol molecules in the CZTS precursor solution associate through hydrogen bonding,which improved the colloidal size and uniformity of the colloidal particles and the thermogravimetric characteristics of the precursor solution,thereby reducing the impurity content and carbon residue in the CZTS precursor film.The increase of the built-in potential and minority carrier lifetime in the absorber layer was beneficial to enhance the carrier separation and charge transfer behavior in the device,significantly improve the carrier transport dynamics in the absorber layer,reduce the carrier recombination rate,and thus significantly improve the quality of the CZTSSe absorber layer.Finally,a CZTSSe photovoltaic device with a certification efficiency of12.07%was obtained.（2）The relationship between pre-annealing conditions and film quality was revealed and the pre-annealing process was optimized.The relationship between stress and film cracking was clarified by exploring the pre-annealing method,and the pre-annealing method of’stepwise heating’was determined.By exploring the pre-annealing temperature,we explained the effects of different crystallization mechanisms on the crystallization quality of the absorber layer and device performance based on different pre-annealing temperatures.Finally,we fabricated the film with good uniformity under the optimal pre-annealing process（280℃）,in which the bulk elements were evenly distributed.The oxygen element was enriched at the edge of the pinhole in the fine grain layer,which can passivate defects and dangling bonds.In addition,based on the high quality of the heterojunction interface,the highest certification efficiency of 12.18%was finally obtained.（3）Aiming at the problem of large open-circuit voltage(V_oc)deficit of CZTSSe solar cells,Ga substitution was introduced to improve the defect characteristics of the absorber layers.It was found that the secondary phase was easy to emerge in the absorber film based on the evaporation method,and it was difficult to effectively control the Ga doping amount and doping effect.The doping method based on the chemical method could suppress Cu_Zn antisite defect in the bulk,thereby reducing its activation energy and defect state density in the absorber layer and lowering the Urbach energy and the(V_oc)deficit of the device.The Ga_Sn antisite defects at the grain boundaries inhibit the generation of deep-level defects,increase the band bending at the grain boundaries,promote the separation and transportation of carriers,and inhibit the recombination of photogenerated carriers.Finally,a V_oc of 522 m V was obtained on the device with an efficiency of more than 12%at 2%Ga doping concentration.（4）The universal law of the synergistic doping effect of double cations was established,and the key factor limiting the efficiency improvement of CZTSSe thin film solar cells was revealed.Firstly,the effect of Ag and Ge cations doping alone was investigated.It was found that they can greatly increase the device V_oc but at the expense of narrowing the light absorption range of the device and sacrificing the current density.In terms of co-doping of double cations,the co-doping of Ge and Ga could greatly improve the V_oc of the devices to more than 530m V,exceeding the V_oc of the world record device（529 m V）.The effective improvement of V_oc by Ge and Ga co-doping confirms that Sn-related defects in the absorber layer are the key factors restricting the quality of the absorption layer,which points out the direction for the subsequent efficiency improvement of CZTSSe thin film solar cells.This dissertation contains 94 figures,20 tables,and 202 references.